{%extends "base/page.html"%}
{%block title%}Backend Flowchart{%endblock%}
{%block contents%}

<h1>Backend Flowchart <i class="fas fa-code-branch"></i></h1>

<center>
<h3><em>Click on an item</em> to see more detail or look at the full
<a href="https://wiki.postgresql.org/wiki/Backend_flowchart">index.</a></h3>

<p><img src="../../../media/img/developer/backend/flow.gif" usemap="#flowmap" alt="flowchart" />

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<br />

<p>A query comes to the backend via data packets arriving through TCP/IP
or Unix Domain sockets. It is loaded into a string, and passed to the <a
href="https://wiki.postgresql.org/wiki/Backend_flowchart#parser">parser,</a>
where the lexical scanner, <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/scan.l">scan.l,</a>
breaks the query up into tokens(words). The parser uses <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/gram.y">gram.y</a>
and the tokens to identify the query type, and load the proper
query-specific structure, like <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/parsenodes.h">CreateStmt</a>
or <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/parsenodes.h">SelectStmt.</a></p>

<p>The statement is then identified as complex (<em>SELECT / INSERT /
UPDATE / DELETE</em>) or simple, e.g <em>CREATE ROLE, ANALYZE,</em>
etc.  Simple utility commands that do not require the executor are processed by statement-specific
functions in the <a href="https://wiki.postgresql.org/wiki/Backend_flowchart#commands">commands</a> module.
Complex statements require more handling.</p>

<p>The parser takes a complex query, and creates a <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/parsenodes.h">Query</a>
structure that contains all the elements used by complex queries.
Query.jointree holds the <em>FROM</em> and <em>WHERE</em> clauses, which is filled
in by <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/parse_clause.c">transformFromClause()</a> and
<a href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/parse_clause.c">transformWhereClause().</a>
Each table referenced in the query is represented by a <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/parsenodes.h">RangeTblEntry,</a>
and they are linked together to form the <em>range table</em> of the
query, which is generated by <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/parse_clause.c">transformFromClause().</a>
Query.rtable holds the query's range table.</p>

<p>Certain queries, like <em>SELECT,</em> return columns of data. Other
queries, like <em>INSERT</em> and <em>UPDATE,</em> specify the columns
modified by the query. These column references are converted to <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/primnodes.h">TargetEntry</a>
entries, which are linked together to make up the <em>target list</em> of
the query. The target list is stored in Query.targetList, which is
generated by <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/parser/parse_target.c">transformTargetList().</a></p>

<p>Other query elements, like aggregates(<em>SUM()</em>), <em>GROUP
BY,</em> and <em>ORDER BY</em> are also stored in their own Query
fields.</p>

<p>The next step is for the Query to be modified by any
<em>VIEWS</em> or <em>RULES</em> that may apply to the query. This is
performed by the <a href="https://wiki.postgresql.org/wiki/Backend_flowchart#rewrite">rewrite</a>
system.</p>

<p>The <a
href="https://wiki.postgresql.org/wiki/Backend_flowchart#optimizer_path">optimizer</a>
uses the Query structure to determine the best table join order and join
type of each table in the RangeTable, using Query.jointree(<em>FROM</em>
and <em>WHERE</em> clauses) to consider optimal index usage.</p>  The <a
href="https://wiki.postgresql.org/wiki/Backend_flowchart#optimizer_path">path</a>
module  then generates an optimal <a
href="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/nodes/plannodes.h">Plan,</a>
which contains the operations to be performed to execute the query.</p>

<p>The Plan is then passed to the <a
href="https://wiki.postgresql.org/wiki/Backend_flowchart#executor">executor</a> for execution, and the
result returned to the client. The Plan is actually as set of nodes,
arranged in a tree structure with a top-level node, and various
sub-nodes as children.</p>

<p>There are many other modules that support this basic
functionality. They can be accessed by clicking on the
flowchart.</p>

{%endblock%}
